This invention relates to a multilayered catalyst system for the dehydrogenation of hydrocarbon feed streams, particularly C3 and C4 hydrocarbon feed streams, and processes for the manufacture and use of the catalyst system. This invention more specifically relates to a multi-layered catalyst system containing at least a first and a second layer of catalysts, wherein the first layer includes a chromia/alumina catalyst stabilized with a zirconium additive, wherein the alumina is eta alumina, and wherein the second layer includes a chromia/alumina catalyst, wherein the alumina is also eta alumina, but which does not contain a zirconium additive, which catalyst system is particularly useful for the dehydrogenation of an hydrocarbon feed stream, particularly a C3 and C4 alkane hydrocarbon feed stream, and processes for the manufacture and use of the catalyst system.
Alkane dehydrogenation is a recognized process for production of a variety of useful hydrocarbon products, such as isobutylene for conversion to MTBE, isooctane and alkylates to supplement and enrich gasolines and propylene for use in the polymer industry. There are several current catalytic processes useful for catalytic dehydrogenation of light alkanes, including Süd-Chemie CATOFIN® processes, the Linde/BASF process, UOP's OLEFLEX® process, Phillips' STAR™ process and the Snamprogetti-Yarsintee process. The catalysts that are used in these processes are manufactured from different types of catalytic materials. For example, the Süd-Chemie CATOFIN® processes utilize chromia-alumina catalysts.
Chromia-alumina dehydrogenation catalyst technology has been in use for over fifty years. In one example, GB 942,944 discloses a dehydrogenation catalyst for the dehydrogenation of aliphatic hydrocarbons having three to five carbon atoms. The catalyst disclosed was prepared by dehydrating an aluminum trihydrate composition comprising 60 to 100 percent beta alumina trihydrate, heating the resulting dehydrated alumina with steam to adjust its surface area to a range of 100 to 200 m2/g, depositing chromium oxide from about 10 to about 25 percent as Cr2O3 onto the resulting alumina carrier and steam treating the resulting catalyst at an elevated temperature.
The stability of a dehydrogenation catalyst plays an important role in the overall efficiency of the dehydrogenation process. Because of the high temperatures at which the catalytic dehydrogenation procedure is conducted, the life expectancy of the catalysts is often limited. Thus, improving the thermal stability of the catalysts translates into longer catalyst life, allowing for longer catalyst utilization and ultimately resulting in lower consumption of the catalysts during the dehydrogenation process.
One proposed method of stabilizing chromia-alumina dehydrogenation catalysts is by the addition of zirconia as disclosed in U.S. Pat. No. 2,374,404.
There are a number of different types of alumina that are available for use as the support for dehydrogenation catalysts. Conventionally, mid to high surface area gamma alumina has been the preferred choice as the carrier for such catalysts. In particular, gamma alumina has been preferred over eta alumina as the carrier for dehydrogenation catalysts.
This preference for gamma alumina over eta alumina for catalysts is not surprising because gamma alumina is generally perceived as having greater thermal stability than eta alumina. In fact, gamma alumina has become the standard alumina utilized for commercial catalysts. (The market has accepted this principle as gamma alumina is readily available in the market while eta alumina is sparsely available, if at all.)
Although dehydrogenation catalysts prepared from chromia-alumina catalysts have been extensively employed for many years, there are still opportunities for improvement, especially to improve the thermal stability. Even when these catalysts are stabilized by the addition of an additive, such as a zirconium or a silicon compound, they can still show limited stability because of the severity of the operating conditions, particularly at the high temperatures normally utilized during the dehydrogenation procedure.
Accordingly, it is an object of this invention to disclose an improved catalyst system for the dehydrogenation of hydrocarbon feed streams wherein the improved catalyst system includes at least two layers of different catalysts contained within the system through which the hydrocarbon feed stream passes in sequence.
These and other objects can be obtained by the catalyst systems, the processes for the preparation of the catalyst systems and the processes of use of the catalyst systems for the dehydrogenation of hydrocarbons which are disclosed by the present invention.